This invention relates to radiation measuring apparatus and more particularly to fluorescence spectrophotometers of the type in which a sample is irradiated with light of one wavelength and its emission spectrum is observed through the use of a monochromator and a detection system. As used herein and in the appended claims, the term "light" includes not only visible light but also radiation having wavelengths longer and shorter than the visible spectrum.
In the measurement of fluorescence and exitation spectra it is customary to illuminate a sample with monochromatic light from an intense source and to observe the light emitted by the sample with a monochromator and a photoelectric detection system. Either the excitation or the emission wavelength may be scanned to record the intensity of the spectrum as a function of excitation or emission wavelength.
Heretofore, radiation measuring apparatus of the foregoing type exhibited certain disadvantages. One of the more significant problems was the comparatively low intensity of the output signal particularly in measuring the spectra of dilute materials. In the usual form of apparatus a magnified image of the light source was focused on the entrance slit of the excitation monochromator, and a reduced image of the exit slit was focused on the sample by means of a first optical system. Fluorescence from the sample was collected by a second optical system and was focused on the entrance slit of an emission monochromator such that the signal at the exit slit of this latter monochromator was proportional to the intensity of the light at the selected wavelength. Attempts to increase the intensity of the signal commonly included a reduction in height of the image of the excitation monochromator's exit slit. These attempts were only partially successful, however, and the measured intensity continued to be insufficient to obtain readings of the desired accuracy for low intensity samples.